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Synthesis of LiNi0.6Co0.2Mn0.2O2 from mixed cathode materials of spent lithium-ion batteries
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Synthesis of LiNi0.6Co0.2Mn0.2O2 from mixed cathode materials of spent lithium-ion batteries Wei Chu, YaLi Zhang *, Xia Chen, YaoGuo Huang, HongYou Cui, Ming Wang, Jing Wang School of Chemistry and Chemical Engineering, Shandong University of Technology, 255049, Zibo, Shandong, China
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� A closed loop process for recycling power batteries is developed. � Instead of recycling the single element, the ternary precursor is prepared directly. � Lithium ions in the filtrate are precipi tated into lithium carbonate and reused. � The recycling process can handle mixed cathode materials.
A R T I C L E I N F O
A B S T R A C T
Keywords: Recycle Resynthesis Electrochemical performance Spent lithium-ion batteries
With the aggravation of resource shortage and environmental problems, the disposal of spent lithium-ion bat teries becomes crucial. In this work, a sustainable closed-loop route to recycle mixed cathode materials from spent lithium-ion batteries is proposed. The sulfuric acid and H2O2 were used to leach mixed cathode powders, and parameters such as temperature, acid concentration, and time were investigated. The leaching efficiency of each metal was observed to be above 99%. The ternary cathode precursor was directly prepared from leachate. Li2CO3 was recovered from the residual solution by adding saturated Na2CO3. The mixture of precursor and Li2CO3 was roasted to resynthesize ternary cathode materials (LiNi0.6Co0.2Mn0.2O2). The characterization of LiNi0.6Co0.2Mn0.2O2, as well as its electrochemical performance was investigated. LiNi0.6Co0.2Mn0.2O2 possesses a layered structure and shows excellent cycling stability and rate capacity. The capacities at the first charge and discharge at 0.2C are 196.26 mA h⋅g 1 and 180.072 mA h⋅g 1, respectively. The coulombic efficiency after the second cycle is observed to be about 99%.
1. Introduction Lithium-ion batteries (LiNixCoyMnzO2 [1] or LiFePO4 cells [2] as cathode materials; graphite or vanadate-based material as anode ma terial [3]) are being widely applied to energy storage devices and electric vehicles. The amounts of applied ternary cathode materials
exceed LiFePO4 owing to their high capacity, improved cycling stability, and enhanced security. Currently, LiNi1-x-yCoxMnyO2 is synthesized mainly by using NiSO4⋅6H2O, CoSO4⋅7H2O, and MnSO4⋅H2O [4–6]. With the wide application of ternary cathode materials in lithium-ion batteries, the demand for Ni, Co, and Li has increased significantly. Due to the limited resources of metals, the cost of them fluctuates
* Corresponding author. E-mail address: [email protected] (Y. Zhang). https://doi.org/10.1016/j.jpowsour.2019.227567 Received 26 August 2019; Received in revised form 13 November 2019; Accepted 4 December 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Wei Chu, Journal of Power Sources, https://doi.org/10.1016/j.jpowsour.2019.227567
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considerably, especially, the price of cobalt has risen sharply [7], and they are not renewable materials. The widespread application of lithium-ion batteries leads to a large amount of spent LIBs, and they have dual properties of environmental risk and resource value. Besides, they are hazardous due to the high content of multiple heavy metals and fluorine-containing electrolytes. On the other hand, the large amount of heavy metals, including Li, Ni, Co, Mn, Al, and Cu, can be recycled, which have much resource value and thus attracted researchers to develop various technologies to minimize risk and obtain maximum benefits [8]. If they are not recycled suitably, then the open-loop industrial cycle is not sustainable. There are two ways to dispose spent LIBs, ladder-form utilization, and recycling. Since cathode electrode materials such as LiCoO2 possess a higher metal value, recycling is the better way compared to using them for cheaper energy storage in the direction of completely utilizing them in terms of value. Also, for the used LIBs or damaged LIBs, recycling is the final approach. Academia and industries have addressed immensely to recycle Li-ion batteries [9]. In the recycling technologies, hydrometallurgy with leaching [10] and extraction [11] as the primary steps are attracted intense attention than pyrometallurgy considering environmental pro tection. In the leaching process, the mineral acids such as H2SO4 [12, 13], HCl [14–16] and HNO3 [17,18] and organic acids such as formic acid [19], DL-malic acid, ascorbic acid [20], oxalic acid [21], meth anesulfonic acid [22] and trichloroacetic acid [23] were applied as leaching agents. Table 1S summarizes some of the operational condi tions, efficiency, and the types of cathode materials for the leaching of spent Li-ion batteries using different acid media. The mineral acids are proved to possess better leaching efficiency than organic acids, as dis played in Table 1S. Also, mineral acids readily leach all the metals efficiently from the cathode scrap, yet organic acids leach the targeted metals selectively, where the leaching efficiency of cobalt is found to be lower. To separate the cathode material from the aluminum foil and minimize the leaching of aluminum, Gao et al. [19] used formic acid to leach the cathode scrap of LIBs based on the selectivity of organic acids. The leaching efficiency of Li could reach 99.93%, yet Ni, Co, and Mn could only reach about 85%, and also 20% Al could be leached. The organic acids usually suffer from the low flash point, effumability, and high price, and thus, it becomes difficult to apply them in the industrial mass production. Therefore, the advantages of mineral acids are more prominent. In the frequently used mineral acids, HCl is volatile, and Cl is harmful to the environment and the human body. HNO3 shows strong oxidation, corrosivity, and instability. H2SO4 is often the first choice of industries because of its stability, price advantage, and high efficiency. The properties of Ni, Co, and Mn are considerably similar, and thus, they are hard to separate. Pranolo et al. [24] used the mixed Ionquest 801 and Acorga M5640 as extractant to separate Co and Li, following which Cyanex 272 was employed as a second solvent in the second stage that led to obtaining pure cobalt and lithium products. Fernandes [25] used Alamine 336 to extract Co, where 60% of Co (II) was extracted in the first stage, and 93.6% of Co (II) along with 2.8% of Ni (II) were extracted in the second stage. Extraction is possibly the only method through which a thorough separation of Ni, Co, and Mn could be ach ieved [26]. It is well known that these extraction agents are too expensive, the followed process is time-consuming, and abundant wastewater is produced. Therefore, it is necessary to develop a new methodology to recycle mixed cathode materials successfully. Qina [27] and Li [20] reported that LiNi1/3Co1/3Mn1/3O2 could be resynthesized directly in the leached solution. Among all the cathode materials, LiNi0.6Co0.2Mn0.2O2 is being recognized as a promising candidate on account of excellent diffusivity of lithium-ion with less dependence, higher capacity, and better electrochemical performance than LiNi1/3 Co1/3Mn1/3O2 [13]. Because of the limited resources of cobalt, LiNi0.6 Co0.2Mn0.2O2 might eventually replace LiNi1/3Co1/3Mn1/3O2. The researchers pay close attention to recover metal from LiCoO2 [28,29] or LiNi0.3Co0.3Mn0.3O2 as the single cathode material. In fact,
cathode materials in commercial Li-ion batteries also include LiMn2O4 [30], LiNiO2, LiNi1-x-yCoxMnyO2, LiFePO4, etc. With large-scale appli cations of powered batteries on electric cars and the development of new chemistry, the composition and proportion of various components vary significantly. Previous methods cannot effectively recycle Li-ion batte ries with different chemistry. If each category of cathode material has a separate recovery system, it takes a great deal of space and equipment. Therefore, it is necessary to transform the methodology and exploit a technology to dispose of most of the cathode materials rather than just only one of them concerning efficiency and effectiveness. Four aspects, the diversity of objects, leaching efficiency, closedmaterials-loop, and promising cathode materials, are crucial to be considered in the development of an effective near-to-industry process for the sustainable recycling of spent LIBs. In this context, the following aspects are focused in this study: 1) the effects of different factors on leaching the mixture of cathode materials; 2) characterization of the recycled products (Li2CO3); 3) preparation of cathode materials (LiNi0.6Co0.2Mn0.2O2) as well as their electrochemical performance. 2. Experimental 2.1. Materials and reagents Mixed cathode materials including LiCoO2, LiMn2O4, LiNiO2, and LiNixCoyMnzO2 (x:y:z ¼ 1:1:1, 5:2:3, 4:2:4, and 6:2:2) were employed in the recycle process. Ternary cathode material does not contain iron, and the properties of LiFePO4 are quite different from others as LiFePO4 cells with orthorhombic olivine-type structure have low extraction efficiency for metals [13], and thus it is out of range. H2SO4, NaOH, Na2CO3, NH3⋅H2O (50 wt%), and H2O2 (30 wt%) were the employed analytical reagents. 2.2. Characterization The mixture of spent cathode scrap was roasted at 500 � C for 40 min, cooled to room temperature, and then broken down on the crusher. They were sieved through 325 mesh, whereby the Al foil and cathode material powders were separated. The active material powders were dissolved in the solution of HNO3:HCl (1:3, v/v) to determine the contents of metals by atomic absorption spectroscopy (AAS, TAS-990 F). The metal con tents were found to be 16.90% nickel, 6.63% cobalt, 37.8% manganese, 6.5% lithium, and 0.79% aluminum. 2.3. Leaching of mixed cathode scrap The leaching experiments were conducted in a 1000 mL threenecked and round-bottomed Pyrex reactor with an auto temperature modulator. A reflux condenser was set-up in one bottleneck to avoid the loss of water by evaporation. A water-heating bath with a mechanical stirrer was used to control the temperature. A measured amount of active material powder and a known concentration of H2SO4 and H2O2 solution were added to the reactor and allowed to reach thermal equi librium. The effects of key parameters, i.e., temperature, acid concen tration, and time, were considered individually to ascertain the optimum leaching conditions. After reacting for a preset period, the mixture (leachate and residue) was filtered immediately and washed three times. 2.4. Synthesis and characterization of ternary cathode precursor The precursor was synthesized by co-precipitation. The molarity of Li, Ni, Mn and Co was tested by AAS, and Cu, Al, and Fe were determined by ICP. The contents of the main elements in the leachate are listed in Table 2S, in which the concentration of Al, Fe, and Cu are 0.457, 3.214, and 0.859 ppm, respectively. The molar ratio of Ni:Mn:Co was adjusted to the prescribed proportion by adding NiSO4⋅6H2O and CoSO4⋅7H2O, 2
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and the molarity was again confirmed by AAS. 100 ml 0.5 mol/L ammonia solution was added as the base solution into the reactor, and N2 was passed through the reactor to prevent Mn2þ from being oxidized. When they were heated up to 60 � C, 2 mol/L of NH3⋅H2O and 4 M NaOH were added dropwise into the reactor using a peristaltic pump, and pH was controlled at the desired value by a pH controller. The reaction continued for 6 h and aged for 12 h. The stirring was continued at 800 rpm throughout the process. The slurry was filtered, and filter cake (ternary cathode precursor) was washed until the pH of the washed solution was 7, and then dried in a vacuum oven at 120 � C for 12 h. The ternary cathode precursor was characterized using XRD and SEM.
Organic acids are less corrosive to equipment and are environmentally friendly [34–39]. In this study, acetic acid, tartaric acid, and oxalic acid were used as leaching agents. The results, as shown in Fig. 1S, among Ni, Co, Mn, and Li, the leaching of Li is easier compared to others due to its free state in the layered structure [40,41]. Mn and Li can be selectively extracted efficiently by acetic acid, yet the leaching efficiency of Ni and Co is poor. Except for Li, other metals were scarcely leached by tartaric and oxalic acids. However, the four metals can be easily leached by sulfuric acid, and the leaching efficiency could reach a satisfactory value. Thus, to maximize the recovery of valuable metals, sulfuric acid is the best choice. 3.1.2. Effect of temperature Fig. 2S shows the effect of temperature on the leaching process. The leaching of nickel is mainly affected by temperature. With an increase in temperature, the leaching efficiency of nickel rises linearly. Whereas, the leaching efficiency of other metals varies slightly with temperature. At 90 � C, the leaching efficiency of all the metals could reach above 99%.
2.5. Precipitation and characterization of lithium carbonate After the co-precipitation of Ni, Co, and Mn, the solution was concentrated to improve the concentration of lithium and obtain a higher recovery rate. After the concentration of leachate, the impurity of aluminum was found to be 11 mg/L. When the lithium ions in the so lution were concentrated to about 40 g/L, a specific amount of Na2CO3 (1.2 times the theoretical dosage) was added slowly to the solution with intense mechanical stirring [12]. Since the solubility of Li2CO3 decreases with temperature, the precipitation of lithium carbonate was carried out at 95 � C. After 20 min, the slurry was filtered immediately, and the precipitate was washed three times by boiling ultrapure water to remove the residual substances present on the Li2CO3 solid. The remaining filtrate contained a large amount of Naþ and SO24 , and the product Na2SO4 was obtained through evaporation and concentration. No chemical waste was generated during the above precipitation process, which agrees with the core concept of recycling from spent materials to new materials. The obtained materials were characterized using XRD and SEM. The purity of lithium carbonate was tested by ICP-OES, as shown in happsec1.
3.1.3. Effect of duration of leaching Increasing the leaching time is beneficial for the leaching process. As shown in Fig. 3S, when the reaction was conducted over 2.5 h, the leaching efficiencies could reach above 99% under the optimized con ditions. Additional experiments were conducted to obtain leach liquor and verify the experimental results. For further studies, a stock solution was prepared on the synthesis of ternary cathode precursor and the re covery of Li2CO3. 3.1.4. Effect of concentration of acid As the concentration of acid used in the leaching experiment in creases, the leaching efficiency of all four metal cations also gradually increases. From the results, as shown in Fig. 4S, it can be noted that when the acid concentration reaches 2.5 M, the recovery rates of all four metal cations are above 99%.
2.6. Resynthesis and electrochemical property test of LiNi0.6Co0.2Mn0.2O2
3.2. Recovery of Li2CO3
Ternary cathode precursor was thoroughly mixed with Li2CO3 as prepared in the section of 2.5 to carry out the closed-materials-loop in this process. The mixture was heated at 500 � C for 4 h and 800 � C for 15 h under O2 atm for Li2CO3 to burn fully, and the amount of Li2CO3(nLi = nNi0:6Co0:2Mn0:2ðOHÞ2 ¼ 1:05 was strictly controlled. Both the heating and � cooling were at a rate of 10 C/min. The active cathode powders of LiNi0.6Co0.2Mn0.2O2 were attained by the above method. The phase was identified by XRD (Bruker AXSD8 Advance), and the morphologies of the samples were analyzed by SEM (Hitachi SU8010). Moreover, energy dispersive X-ray spectroscopy (EDS) mapping was carried out using OXFORD 7426, which was an attachment of SEM [31]. CR2032 coin cells were used to study the electrochemical perfor mance of active cathode powders. The proportion of active material, acetylene black, and PVDF binder in the cathode electrode is 8:1:1. Then, an appropriate amount of NMP was added to the mixture and stirred at 400 rpm for 2 h to mix thoroughly. The electrolyte is 1 M LiPF6, and the anode is metallic lithium. Electrochemical tests were carried out between 2.5 and 4.5 V at 25 � C. Cyclic voltammetry (CV) and Electro chemical impendence spectroscopy (EIS) tests were performed on an electrochemical workstation (CHI).
Li2CO3 was recovered based on the method as mentioned above (section 2.4). The SEM image and XRD of Li2CO3 are shown in Fig. 1, and it could be noted that Li2CO3 demonstrates a regular rectangular shape. According to the results of XRD, the corresponding accuracy of the diffraction intensity of each peak reached more than 99.9%, and Table 3S shows the mass fraction of Li2CO3 is 99.96%; thus, the purity of Li2CO3 reaches the standard battery grade.
3. Results and discussion 3.1. Leaching 3.1.1. Selection of leaching agent The selection of the leaching agent mainly considers the following aspects: economic efficiency, environmental privilege, and leaching ef ficiency. Sulfuric acid is cheap and has a good leaching effect on most of the metals, and thus, it is the first choice of inorganic acids [32,33].
Fig. 1. SEM image and XRD of Li2CO3. 3
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3.3. Characterization of ternary cathode precursor and LiNi0.6Co0.2Mn0.2O2
electrolyte interface (SEI) was formed on the cathode surface during the first charge and discharge cycle. SEI could effectively prevent the solvent molecules from passing, and Liþ could be freely embedded and extracted through SEI. According to the charge and discharge cycle at 0.5C, 1C, and 3C, the LiNi0.6Co0.2Mn0.2O2 has excellent cycling stability. The reason for the decline in the capacity retention rate could be that when the cycle progresses, the internal oxidation-reduction reaction becomes intense, and the polarization aggravates. Partially deintercalated Liþ cannot be re-embedded in time, resulting in a loss of reversible capacity [30]. The rate capacity of LiNi0.6Co0.2Mn0.2O2 was evaluated at 25 � C (Fig. 5 (e)). The cell was charged/discharged at 0.2C, 0.5C, 1C, 3C, and 5C for every five cycles. Fig. 5 (e) demonstrates the excellent rate capacity of LiNi0.6Co0.2Mn0.2O2. The cyclic voltammetry characteristics of the first three cycles of LiNi0.6Co0.2Mn0.2O2 are shown in Fig. 5S (a), and it could be observed that the oxidation peak of the first cycle shifts to the right, indicating that the SEI was formed in the first charging process [45]. The formed SEI led to irreversible capacity loss. The oxidation peak appeared at 3.95 V during the first cycle corresponds to a less obvious voltage platform of about 3.9 V in the first charge/discharge curve (Fig. 5S (b)). However, the oxidation/reduction peaks in the second and third cycles were overlapping, indicating that the structure of the material tends to be stable, and the ratio of Liþ insertion and extraction is close to 1 so that the material has excellent cycling stability. EIS of LiNi0.6Co0.2Mn0.2O2 was measured at room temperature, and the results are shown in Fig. 6, which includes the equivalent circuit performed to fit the Nyquist plot. In the impedance spectrum, there is one semicircle in the high-frequency region and one diagonal line in the low-frequency region. The intercept of the impedance spectrum and the Z0 semicircle in the high-frequency region represent the charge transfer impedance and capacitance in the electrochemical reaction (CPE1/Rr þ Rw), which indicates that Liþ could be extracted/embedded between the electrode material and the electrolyte interface. The smaller the value, the easier the reaction. The slant line in the low-frequency region is the Warburg impedance, which revealed the diffusion resistance of Liþ in the solid phase materials. The slope of the Warburg impedance is greater than 1, signifying the existence of the diffusion of the barrier layer [46]. The barrier layer is a solid electrolyte interface (SEI) generated on the negative electrode surface, which is consistent with the results as shown in Fig. 5 (a). SEI is favorable for the diffusion of lithium-ion so that the dynamic diffusion rate of Liþ at the interface is significant, and the electrochemical performance is excellent.
The SEM image of the precursor, as shown in Fig. 2a, indicates that the morphology is characterized by spherical agglomerates with a small uniform size of 6 μm. Through calcination, the precursor is combined with Li2CO3 to form LiNi0.6Co0.2Mn0.2O2. The SEM image (Fig. 2b) ex hibits the regular polygon particles of LiNi0.6Co0.2Mn0.2O2 with a size of 1 μm. The SEM-EDS spectrum (Fig. 3 (a)) of the precursor indicates the characteristic peaks of O, Ni, Co, and Mn, and no other peaks could be detected, which proved the purity of the precursor. The contents of metal elements in the ternary cathode precursor are presented in Table 4S, which indicated that the molar ratio of nickel and cobalt (Ni/ Co) is 3.13, and that of nickel and manganese (Ni/Mn) is 2.99, so Ni/Co/ Mn is close to 6:2:2. In the ternary cathode precursor, no Li was deter mined, and the contents of Cu, Fe, and Al were 0.796, 3.214, and 0.859 ppm, respectively. Fig. 3(c–f) shows the distribution of all the four ele ments. It could be noted that every element is evenly distributed across the micrograph. The molar ratio of LiNi0.6Co0.2Mn0.2O2 was calculated to be about 6:2:2 according to the atomic percentages of O, Ni, Co, and Mn (Table 5S), which agrees with the theoretical elemental composition of the precursor. The XRD pattern of LiNi0.6Co0.2Mn0.2O2 is shown in Fig. 4. It could be observed that the peaks perfectly match with the standard PDF card. Every diffraction peak belongs to the characteristic peak of α-NaFeO2 type with the space group of R3m, and no impurity peak was detected in the sample. This indicates the high purity of the obtained material. Different parameters of LiNi0.6Co0.2Mn0.2O2 crystal were calculated and have been shown in Table 6S. The c/a value of LiNi0.6Co0.2Mn0.2O2 is 4.932, and there are two significant bifurcations at the peaks of 006/102 and 108/110. All these suggest the presence of an excellent layered structure of LiNi0.6Co0.2Mn0.2O2 [42]. The intensity ratio (R) of I (003) and I (104) is a sensitive parameter to determine the degree of cation distribution in the crystal lattice and to measure the degree of cation mixing in the crystal of the material. It can be considered that the cation mixing is relatively light when R � 1.2. The larger the R-value, the lower the degree of cation mixing, and the more favorable the migration of lithium ions [43]. The R-value of LiNi0.6 Co0.2Mn0.2O2 is 1.13, which indicates that a part of 3a Liþ was replaced by 3b Ni2þ, and exhibits a significant relationship with the high content of Ni, and the preparation of cathode with high nickel content requires a more rigorous process [44]. 3.4. Electrochemical properties of the resynthesized LiNi0.6Co0.2Mn0.2O2
3.5. Mechanism of the synthesis of ternary cathode precursor
The cycling stabilities of LiNi0.6Co0.2Mn0.2O2 were measured at the rate of 0.2C, 0.5C, 1 C, and 3 C in a potential window ranging from 2.7 to 4.2 V (Fig. 5(a–d)). The specific capacity at the first charge and discharge at 0.2C were 196.26 mA h.g 1, and 180.072 mA h.g 1, respectively, and the coulombic efficiency was about 90%. A solid
The leaching solution contains a large amount of Ni2þ, Co2þ, and Mn , and when NaOH solution is added to the solution, the following reactions occur [47]: 2þ
Fig. 2. SEM images of the ternary cathode precursor (a) and LiNi0.6Co0.2Mn0.2O2 (b). 4
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Fig. 3. SEM-EDS images of LiNi0.6Co0.2Mn0.2O2 and elemental mapping with the corresponding mappings of single elements.
Fig. 4. XRD pattern of ternary cathode material.
Ni2þ þ 2OH ⇋ Ni(OH)2, ksp ¼ 2.0 � 10
15
Co2þ þ 2OH ⇋ Co(OH)2, ksp ¼ 1.09 � 10 Mn
2þ
þ 2OH ⇋ Mn(OH)2, ksp ¼ 1.9 � 10 2
,
13
(1)
,
(2)
,
(3)
15
In the process of the synthesis of precursor, the nickel and cobalt ions at first interact with NH3⋅H2O in a basic solution, and form Ni(NH3)2þ i (i ¼ 0, 1, 2, 3, 4, 5, and 6) and Co(NH3)2þ j (j ¼ 0, 1, 2, 3, 4, 5, and 6), yet the manganese ammonia complex ion is Mn(NH3)2þ k (k ¼ 0, 1, and 2). This reduces the concentration of metal ions in the solution and effectively prevents the rate of formation of M(OH)2 crystal nuclei. Subsequently, when ammonia water is added simultaneously with NaOH, an equilib rium system of Ni(II)–Co(II)–Mn(II)–NHþ 4 -NH3-H2O exists in the reactor. Since the solubility-product constant (Ksp) of manganese is much larger than that of nickel and cobalt, it balances the deficiency of manganese and NH3 complexation as compared to nickel and cobalt. Therefore, controlling [NH3]T ([NH3]T: total concentration of NH3) and pH can adjust the precipitation rates of Ni2þ, Co2þ, and Mn2þ to reach similar ones. P. Yang [49] calculated the thermodynamics of the system to demonstrate that the amount of ammonia affects the equilibrium of Eqns. (1)–(3). Thus, a high-performance ternary cathode precursor can be synthesized by adjusting the pH value and concentration of ammonia to effectively control the rate of precipitation and morphology of the product. With the continuous addition of NaOH, the pH of the solution in the reactor gradually rises. When the pH reaches a specific value, a large amount of OH in the solution reacts with M(NH3)2þ n complex ion to generate the M(OH)2 crystal nuclei, and the nucleus continuously grows. Finally, a spherical precursor product is obtained. The mechanism of the synthesis of ternary cathode precursor is presented in Fig. 7. As shown in Fig. 7, the mixed cathode materials of spent lithium-ion batteries were leached using sulfuric acid solution, and the ternary cathode precursor was synthesized by co-precipitation in the presence of complexing agent (ammonia water) and the precipitant sodium hydroxide.
2þ
Ni þ and Co start to precipitate at the pH values of 6.7 and 6.6, respectively, yet Mn2þ begins to precipitate at 7.8 [48]. The pH of the leaching solution is about 1.5, and the ksp values of Ni(OH)2 and Co (OH)2 differ significantly from that of Mn(OH)2. If the NaOH solution is directly added into the leaching solution, Ni2þ and Co2þ are preferen tially precipitated, and co-precipitation with Mn2þ to form ternary cathode precursor cannot be achieved. At the same time, the rate of precipitation and morphology of the product cannot be accurately controlled. The contrastive experimental results obtained without add ing ammonia water are presented in Fig. 6S(a) and (b). Fig. 6S(a) shows that the ternary cathode precursor has no regular morphology, and the specific capacity at the first charge and discharge at 0.2C (Fig. 6S(b)) were less than that observed in Fig. 5a. Thus, reducing the rate of pre cipitation of nickel-cobalt and realizing the co-precipitation of three metal ions in a strongly acidic solution is an important factor that de cides the quality of the obtained product. It is well known that the ammonia water has a strong ability to complex with nickel, cobalt, and manganese ions, and the reaction is expressed in Eqn. (4), and the sta bility constants (K) [48] are presented in Table 7S. M2þ þ nNH3 ⇋ [M(NH3)n]2þ
3.6. Sustainable recycling process for the spent lithium-ion batteries Based on the severe current environmental protection, sustainable cycling of spent lithium-ion batteries is proposed. The flow chart of this process is shown in Fig. 8. The mixture of cathode scraps could be ob tained after the spent lithium-ion batteries are discharged and dis assembled. Subsequently, the valuable metals and insoluble materials could be separated by leaching. The leaching efficiencies of nickel, co balt, manganese, and lithium could reach more than 99%. Then, the ion concentration ratio of the leachate was adjusted, and the precursor was resynthesized by co-precipitation in the presence of ammonia water and
(4) 5
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Fig. 5. Charging and discharging capacity at 0.2C, 0.5C, 1 C, and 3 C (a–d); discharge capacity at 0.2C, 0.5C, 1 C, 3 C, and 5 C (e).
LiNi0.6Co0.2Mn0.2O2 could meet the industrial requirements. The direct synthesis of the precursor in this process considerably reduced the consumption of raw material, and the rate of loss of metal elements was less than 1%. No other impurities were formed during the leaching process, and no secondary pollution was noted. Na2SO4 was obtained by evaporating the leachate after the recovery of Li2CO3. NH3 and H2O could continue to cycle in the process by condensation. Therefore, this closed-circuit cycle has the prospect of recycling of spent battery and is in line with the strategic requirements of social sustainability. 4. Conclusion In this work, a sustainable closed-loop route to recycle the mixed cathode powders from spent lithium-ion batteries has been presented. LiNi0.6Co0.2Mn0.2O2 was directly resynthesized, and the leaching effi ciencies of Ni, Co, Mn, and Li were above 99% under the optimized conditions. The ternary cathode precursor was prepared by coprecipitation after adjusting the molar ratio of ions. Li2CO3 was ob tained in the presence of a saturated sodium sulfate solution. The characterization of LiNi0.6Co0.2Mn0.2O2 was carried out using SEM and XRD. The SEM presented uniform particle size, and the XRD pattern suggested the presence of a good-layered structure of LiNi0.6 Co0.2Mn0.2O2. The electrochemical properties of the resynthesized LiNi0.6Co0.2Mn0.2O2 were determined by the charge and discharge cycle, cyclic voltammetry characteristics, AC-impedance, and rate capacity.
Fig. 6. Nyquist plot of the cell and equivalent circuit performed to fit the Nyquist plot.
sodium hydroxide. The principal role of ammonia (2:1) is to complex nickel and cobalt to control the rate of precipitation. Liþ in the filtrate was precipitated to form Li2CO3 under the action of Na2CO3, and the mixture of the precursor and Li2CO3 was calcined to obtain LiNi0.6 Co0.2Mn0.2O2. The electrochemical performance of the resynthesized 6
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Fig. 7. The mechanism of the synthesis of ternary cathode precursor.
excellent cycling stability. The cell charge/discharge at 0.2C, 0.5C, 1 C, 3 C, and 5 C for every five cycles also showed an excellent rate capacity. Declaration of competing interest The authors declared that they have no conflicts of interest to this work. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (51974180; 21978158); Shandong Provincial Natural Science Foundation, China (ZR2018MEE011); and SDUT & Zibo City Integration Development Project (2017ZBXC070). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227567. References [1] Y. Li, Y. Bai, C. Wu, J. Qian, G.H. Chen, L. Liu, H. Wang, X.Z. Zhou, F. Wu, Threedimensional fusiform hierarchical micro/nano Li1.2Ni0.2Mn0.6O2 with preferred orientation (110) plane as high energy cathode material for lithium-ion batteries, J. Mater. Chem. 4 (2016) 5942–5951. [2] J.J. Wang, X.L. Sun, Understanding and recent development of carbon coating on LiFePO4 cathode materials for lithium-ion batteries, Energy Environ. Sci. 5 (2012) 5163–5185. [3] S.B. Ni, J.L. Liu, D.L. Chao, L.Q. Mai, Vanadate-based materials for Li-Ion batteries: the search for anodes for practical applications, Adv. Energy Mater. (9) (2019) 1803324. [4] S. Florian, D. Mudit, K. Daniela, T. Michael, H. Ortal, G. Judith, M. Evan, C. Erickson, D. Ghanty, T.M. Boris, A. Doron, Stabilizing nickel-rich layered
Fig. 8. The flow-process diagram.
The specific capacity at the first charge and discharge at 0.2C were 196.26 mA h.g 1 and 180.072 mA h.g 1, respectively, which suggested the formation of a solid electrolyte interface on the cathode surface. The coulombic efficiency could reach 99% after two cycles, which presented 7
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Synthesis of LiNi0.6Co0.2Mn0.2O2 from mixed cathode materials of spent lithium-ion batteries Wei Chu, YaLi Zhang *, Xia Chen, YaoGuo Huang, HongYou Cui, Ming Wang, Jing Wang School of Chemistry and Chemical Engineering, Shandong University of Technology, 255049, Zibo, Shandong, China
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� A closed loop process for recycling power batteries is developed. � Instead of recycling the single element, the ternary precursor is prepared directly. � Lithium ions in the filtrate are precipi tated into lithium carbonate and reused. � The recycling process can handle mixed cathode materials.
A R T I C L E I N F O
A B S T R A C T
Keywords: Recycle Resynthesis Electrochemical performance Spent lithium-ion batteries
With the aggravation of resource shortage and environmental problems, the disposal of spent lithium-ion bat teries becomes crucial. In this work, a sustainable closed-loop route to recycle mixed cathode materials from spent lithium-ion batteries is proposed. The sulfuric acid and H2O2 were used to leach mixed cathode powders, and parameters such as temperature, acid concentration, and time were investigated. The leaching efficiency of each metal was observed to be above 99%. The ternary cathode precursor was directly prepared from leachate. Li2CO3 was recovered from the residual solution by adding saturated Na2CO3. The mixture of precursor and Li2CO3 was roasted to resynthesize ternary cathode materials (LiNi0.6Co0.2Mn0.2O2). The characterization of LiNi0.6Co0.2Mn0.2O2, as well as its electrochemical performance was investigated. LiNi0.6Co0.2Mn0.2O2 possesses a layered structure and shows excellent cycling stability and rate capacity. The capacities at the first charge and discharge at 0.2C are 196.26 mA h⋅g 1 and 180.072 mA h⋅g 1, respectively. The coulombic efficiency after the second cycle is observed to be about 99%.
1. Introduction Lithium-ion batteries (LiNixCoyMnzO2 [1] or LiFePO4 cells [2] as cathode materials; graphite or vanadate-based material as anode ma terial [3]) are being widely applied to energy storage devices and electric vehicles. The amounts of applied ternary cathode materials
exceed LiFePO4 owing to their high capacity, improved cycling stability, and enhanced security. Currently, LiNi1-x-yCoxMnyO2 is synthesized mainly by using NiSO4⋅6H2O, CoSO4⋅7H2O, and MnSO4⋅H2O [4–6]. With the wide application of ternary cathode materials in lithium-ion batteries, the demand for Ni, Co, and Li has increased significantly. Due to the limited resources of metals, the cost of them fluctuates
* Corresponding author. E-mail address: [email protected] (Y. Zhang). https://doi.org/10.1016/j.jpowsour.2019.227567 Received 26 August 2019; Received in revised form 13 November 2019; Accepted 4 December 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Wei Chu, Journal of Power Sources, https://doi.org/10.1016/j.jpowsour.2019.227567
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considerably, especially, the price of cobalt has risen sharply [7], and they are not renewable materials. The widespread application of lithium-ion batteries leads to a large amount of spent LIBs, and they have dual properties of environmental risk and resource value. Besides, they are hazardous due to the high content of multiple heavy metals and fluorine-containing electrolytes. On the other hand, the large amount of heavy metals, including Li, Ni, Co, Mn, Al, and Cu, can be recycled, which have much resource value and thus attracted researchers to develop various technologies to minimize risk and obtain maximum benefits [8]. If they are not recycled suitably, then the open-loop industrial cycle is not sustainable. There are two ways to dispose spent LIBs, ladder-form utilization, and recycling. Since cathode electrode materials such as LiCoO2 possess a higher metal value, recycling is the better way compared to using them for cheaper energy storage in the direction of completely utilizing them in terms of value. Also, for the used LIBs or damaged LIBs, recycling is the final approach. Academia and industries have addressed immensely to recycle Li-ion batteries [9]. In the recycling technologies, hydrometallurgy with leaching [10] and extraction [11] as the primary steps are attracted intense attention than pyrometallurgy considering environmental pro tection. In the leaching process, the mineral acids such as H2SO4 [12, 13], HCl [14–16] and HNO3 [17,18] and organic acids such as formic acid [19], DL-malic acid, ascorbic acid [20], oxalic acid [21], meth anesulfonic acid [22] and trichloroacetic acid [23] were applied as leaching agents. Table 1S summarizes some of the operational condi tions, efficiency, and the types of cathode materials for the leaching of spent Li-ion batteries using different acid media. The mineral acids are proved to possess better leaching efficiency than organic acids, as dis played in Table 1S. Also, mineral acids readily leach all the metals efficiently from the cathode scrap, yet organic acids leach the targeted metals selectively, where the leaching efficiency of cobalt is found to be lower. To separate the cathode material from the aluminum foil and minimize the leaching of aluminum, Gao et al. [19] used formic acid to leach the cathode scrap of LIBs based on the selectivity of organic acids. The leaching efficiency of Li could reach 99.93%, yet Ni, Co, and Mn could only reach about 85%, and also 20% Al could be leached. The organic acids usually suffer from the low flash point, effumability, and high price, and thus, it becomes difficult to apply them in the industrial mass production. Therefore, the advantages of mineral acids are more prominent. In the frequently used mineral acids, HCl is volatile, and Cl is harmful to the environment and the human body. HNO3 shows strong oxidation, corrosivity, and instability. H2SO4 is often the first choice of industries because of its stability, price advantage, and high efficiency. The properties of Ni, Co, and Mn are considerably similar, and thus, they are hard to separate. Pranolo et al. [24] used the mixed Ionquest 801 and Acorga M5640 as extractant to separate Co and Li, following which Cyanex 272 was employed as a second solvent in the second stage that led to obtaining pure cobalt and lithium products. Fernandes [25] used Alamine 336 to extract Co, where 60% of Co (II) was extracted in the first stage, and 93.6% of Co (II) along with 2.8% of Ni (II) were extracted in the second stage. Extraction is possibly the only method through which a thorough separation of Ni, Co, and Mn could be ach ieved [26]. It is well known that these extraction agents are too expensive, the followed process is time-consuming, and abundant wastewater is produced. Therefore, it is necessary to develop a new methodology to recycle mixed cathode materials successfully. Qina [27] and Li [20] reported that LiNi1/3Co1/3Mn1/3O2 could be resynthesized directly in the leached solution. Among all the cathode materials, LiNi0.6Co0.2Mn0.2O2 is being recognized as a promising candidate on account of excellent diffusivity of lithium-ion with less dependence, higher capacity, and better electrochemical performance than LiNi1/3 Co1/3Mn1/3O2 [13]. Because of the limited resources of cobalt, LiNi0.6 Co0.2Mn0.2O2 might eventually replace LiNi1/3Co1/3Mn1/3O2. The researchers pay close attention to recover metal from LiCoO2 [28,29] or LiNi0.3Co0.3Mn0.3O2 as the single cathode material. In fact,
cathode materials in commercial Li-ion batteries also include LiMn2O4 [30], LiNiO2, LiNi1-x-yCoxMnyO2, LiFePO4, etc. With large-scale appli cations of powered batteries on electric cars and the development of new chemistry, the composition and proportion of various components vary significantly. Previous methods cannot effectively recycle Li-ion batte ries with different chemistry. If each category of cathode material has a separate recovery system, it takes a great deal of space and equipment. Therefore, it is necessary to transform the methodology and exploit a technology to dispose of most of the cathode materials rather than just only one of them concerning efficiency and effectiveness. Four aspects, the diversity of objects, leaching efficiency, closedmaterials-loop, and promising cathode materials, are crucial to be considered in the development of an effective near-to-industry process for the sustainable recycling of spent LIBs. In this context, the following aspects are focused in this study: 1) the effects of different factors on leaching the mixture of cathode materials; 2) characterization of the recycled products (Li2CO3); 3) preparation of cathode materials (LiNi0.6Co0.2Mn0.2O2) as well as their electrochemical performance. 2. Experimental 2.1. Materials and reagents Mixed cathode materials including LiCoO2, LiMn2O4, LiNiO2, and LiNixCoyMnzO2 (x:y:z ¼ 1:1:1, 5:2:3, 4:2:4, and 6:2:2) were employed in the recycle process. Ternary cathode material does not contain iron, and the properties of LiFePO4 are quite different from others as LiFePO4 cells with orthorhombic olivine-type structure have low extraction efficiency for metals [13], and thus it is out of range. H2SO4, NaOH, Na2CO3, NH3⋅H2O (50 wt%), and H2O2 (30 wt%) were the employed analytical reagents. 2.2. Characterization The mixture of spent cathode scrap was roasted at 500 � C for 40 min, cooled to room temperature, and then broken down on the crusher. They were sieved through 325 mesh, whereby the Al foil and cathode material powders were separated. The active material powders were dissolved in the solution of HNO3:HCl (1:3, v/v) to determine the contents of metals by atomic absorption spectroscopy (AAS, TAS-990 F). The metal con tents were found to be 16.90% nickel, 6.63% cobalt, 37.8% manganese, 6.5% lithium, and 0.79% aluminum. 2.3. Leaching of mixed cathode scrap The leaching experiments were conducted in a 1000 mL threenecked and round-bottomed Pyrex reactor with an auto temperature modulator. A reflux condenser was set-up in one bottleneck to avoid the loss of water by evaporation. A water-heating bath with a mechanical stirrer was used to control the temperature. A measured amount of active material powder and a known concentration of H2SO4 and H2O2 solution were added to the reactor and allowed to reach thermal equi librium. The effects of key parameters, i.e., temperature, acid concen tration, and time, were considered individually to ascertain the optimum leaching conditions. After reacting for a preset period, the mixture (leachate and residue) was filtered immediately and washed three times. 2.4. Synthesis and characterization of ternary cathode precursor The precursor was synthesized by co-precipitation. The molarity of Li, Ni, Mn and Co was tested by AAS, and Cu, Al, and Fe were determined by ICP. The contents of the main elements in the leachate are listed in Table 2S, in which the concentration of Al, Fe, and Cu are 0.457, 3.214, and 0.859 ppm, respectively. The molar ratio of Ni:Mn:Co was adjusted to the prescribed proportion by adding NiSO4⋅6H2O and CoSO4⋅7H2O, 2
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and the molarity was again confirmed by AAS. 100 ml 0.5 mol/L ammonia solution was added as the base solution into the reactor, and N2 was passed through the reactor to prevent Mn2þ from being oxidized. When they were heated up to 60 � C, 2 mol/L of NH3⋅H2O and 4 M NaOH were added dropwise into the reactor using a peristaltic pump, and pH was controlled at the desired value by a pH controller. The reaction continued for 6 h and aged for 12 h. The stirring was continued at 800 rpm throughout the process. The slurry was filtered, and filter cake (ternary cathode precursor) was washed until the pH of the washed solution was 7, and then dried in a vacuum oven at 120 � C for 12 h. The ternary cathode precursor was characterized using XRD and SEM.
Organic acids are less corrosive to equipment and are environmentally friendly [34–39]. In this study, acetic acid, tartaric acid, and oxalic acid were used as leaching agents. The results, as shown in Fig. 1S, among Ni, Co, Mn, and Li, the leaching of Li is easier compared to others due to its free state in the layered structure [40,41]. Mn and Li can be selectively extracted efficiently by acetic acid, yet the leaching efficiency of Ni and Co is poor. Except for Li, other metals were scarcely leached by tartaric and oxalic acids. However, the four metals can be easily leached by sulfuric acid, and the leaching efficiency could reach a satisfactory value. Thus, to maximize the recovery of valuable metals, sulfuric acid is the best choice. 3.1.2. Effect of temperature Fig. 2S shows the effect of temperature on the leaching process. The leaching of nickel is mainly affected by temperature. With an increase in temperature, the leaching efficiency of nickel rises linearly. Whereas, the leaching efficiency of other metals varies slightly with temperature. At 90 � C, the leaching efficiency of all the metals could reach above 99%.
2.5. Precipitation and characterization of lithium carbonate After the co-precipitation of Ni, Co, and Mn, the solution was concentrated to improve the concentration of lithium and obtain a higher recovery rate. After the concentration of leachate, the impurity of aluminum was found to be 11 mg/L. When the lithium ions in the so lution were concentrated to about 40 g/L, a specific amount of Na2CO3 (1.2 times the theoretical dosage) was added slowly to the solution with intense mechanical stirring [12]. Since the solubility of Li2CO3 decreases with temperature, the precipitation of lithium carbonate was carried out at 95 � C. After 20 min, the slurry was filtered immediately, and the precipitate was washed three times by boiling ultrapure water to remove the residual substances present on the Li2CO3 solid. The remaining filtrate contained a large amount of Naþ and SO24 , and the product Na2SO4 was obtained through evaporation and concentration. No chemical waste was generated during the above precipitation process, which agrees with the core concept of recycling from spent materials to new materials. The obtained materials were characterized using XRD and SEM. The purity of lithium carbonate was tested by ICP-OES, as shown in happsec1.
3.1.3. Effect of duration of leaching Increasing the leaching time is beneficial for the leaching process. As shown in Fig. 3S, when the reaction was conducted over 2.5 h, the leaching efficiencies could reach above 99% under the optimized con ditions. Additional experiments were conducted to obtain leach liquor and verify the experimental results. For further studies, a stock solution was prepared on the synthesis of ternary cathode precursor and the re covery of Li2CO3. 3.1.4. Effect of concentration of acid As the concentration of acid used in the leaching experiment in creases, the leaching efficiency of all four metal cations also gradually increases. From the results, as shown in Fig. 4S, it can be noted that when the acid concentration reaches 2.5 M, the recovery rates of all four metal cations are above 99%.
2.6. Resynthesis and electrochemical property test of LiNi0.6Co0.2Mn0.2O2
3.2. Recovery of Li2CO3
Ternary cathode precursor was thoroughly mixed with Li2CO3 as prepared in the section of 2.5 to carry out the closed-materials-loop in this process. The mixture was heated at 500 � C for 4 h and 800 � C for 15 h under O2 atm for Li2CO3 to burn fully, and the amount of Li2CO3(nLi = nNi0:6Co0:2Mn0:2ðOHÞ2 ¼ 1:05 was strictly controlled. Both the heating and � cooling were at a rate of 10 C/min. The active cathode powders of LiNi0.6Co0.2Mn0.2O2 were attained by the above method. The phase was identified by XRD (Bruker AXSD8 Advance), and the morphologies of the samples were analyzed by SEM (Hitachi SU8010). Moreover, energy dispersive X-ray spectroscopy (EDS) mapping was carried out using OXFORD 7426, which was an attachment of SEM [31]. CR2032 coin cells were used to study the electrochemical perfor mance of active cathode powders. The proportion of active material, acetylene black, and PVDF binder in the cathode electrode is 8:1:1. Then, an appropriate amount of NMP was added to the mixture and stirred at 400 rpm for 2 h to mix thoroughly. The electrolyte is 1 M LiPF6, and the anode is metallic lithium. Electrochemical tests were carried out between 2.5 and 4.5 V at 25 � C. Cyclic voltammetry (CV) and Electro chemical impendence spectroscopy (EIS) tests were performed on an electrochemical workstation (CHI).
Li2CO3 was recovered based on the method as mentioned above (section 2.4). The SEM image and XRD of Li2CO3 are shown in Fig. 1, and it could be noted that Li2CO3 demonstrates a regular rectangular shape. According to the results of XRD, the corresponding accuracy of the diffraction intensity of each peak reached more than 99.9%, and Table 3S shows the mass fraction of Li2CO3 is 99.96%; thus, the purity of Li2CO3 reaches the standard battery grade.
3. Results and discussion 3.1. Leaching 3.1.1. Selection of leaching agent The selection of the leaching agent mainly considers the following aspects: economic efficiency, environmental privilege, and leaching ef ficiency. Sulfuric acid is cheap and has a good leaching effect on most of the metals, and thus, it is the first choice of inorganic acids [32,33].
Fig. 1. SEM image and XRD of Li2CO3. 3
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3.3. Characterization of ternary cathode precursor and LiNi0.6Co0.2Mn0.2O2
electrolyte interface (SEI) was formed on the cathode surface during the first charge and discharge cycle. SEI could effectively prevent the solvent molecules from passing, and Liþ could be freely embedded and extracted through SEI. According to the charge and discharge cycle at 0.5C, 1C, and 3C, the LiNi0.6Co0.2Mn0.2O2 has excellent cycling stability. The reason for the decline in the capacity retention rate could be that when the cycle progresses, the internal oxidation-reduction reaction becomes intense, and the polarization aggravates. Partially deintercalated Liþ cannot be re-embedded in time, resulting in a loss of reversible capacity [30]. The rate capacity of LiNi0.6Co0.2Mn0.2O2 was evaluated at 25 � C (Fig. 5 (e)). The cell was charged/discharged at 0.2C, 0.5C, 1C, 3C, and 5C for every five cycles. Fig. 5 (e) demonstrates the excellent rate capacity of LiNi0.6Co0.2Mn0.2O2. The cyclic voltammetry characteristics of the first three cycles of LiNi0.6Co0.2Mn0.2O2 are shown in Fig. 5S (a), and it could be observed that the oxidation peak of the first cycle shifts to the right, indicating that the SEI was formed in the first charging process [45]. The formed SEI led to irreversible capacity loss. The oxidation peak appeared at 3.95 V during the first cycle corresponds to a less obvious voltage platform of about 3.9 V in the first charge/discharge curve (Fig. 5S (b)). However, the oxidation/reduction peaks in the second and third cycles were overlapping, indicating that the structure of the material tends to be stable, and the ratio of Liþ insertion and extraction is close to 1 so that the material has excellent cycling stability. EIS of LiNi0.6Co0.2Mn0.2O2 was measured at room temperature, and the results are shown in Fig. 6, which includes the equivalent circuit performed to fit the Nyquist plot. In the impedance spectrum, there is one semicircle in the high-frequency region and one diagonal line in the low-frequency region. The intercept of the impedance spectrum and the Z0 semicircle in the high-frequency region represent the charge transfer impedance and capacitance in the electrochemical reaction (CPE1/Rr þ Rw), which indicates that Liþ could be extracted/embedded between the electrode material and the electrolyte interface. The smaller the value, the easier the reaction. The slant line in the low-frequency region is the Warburg impedance, which revealed the diffusion resistance of Liþ in the solid phase materials. The slope of the Warburg impedance is greater than 1, signifying the existence of the diffusion of the barrier layer [46]. The barrier layer is a solid electrolyte interface (SEI) generated on the negative electrode surface, which is consistent with the results as shown in Fig. 5 (a). SEI is favorable for the diffusion of lithium-ion so that the dynamic diffusion rate of Liþ at the interface is significant, and the electrochemical performance is excellent.
The SEM image of the precursor, as shown in Fig. 2a, indicates that the morphology is characterized by spherical agglomerates with a small uniform size of 6 μm. Through calcination, the precursor is combined with Li2CO3 to form LiNi0.6Co0.2Mn0.2O2. The SEM image (Fig. 2b) ex hibits the regular polygon particles of LiNi0.6Co0.2Mn0.2O2 with a size of 1 μm. The SEM-EDS spectrum (Fig. 3 (a)) of the precursor indicates the characteristic peaks of O, Ni, Co, and Mn, and no other peaks could be detected, which proved the purity of the precursor. The contents of metal elements in the ternary cathode precursor are presented in Table 4S, which indicated that the molar ratio of nickel and cobalt (Ni/ Co) is 3.13, and that of nickel and manganese (Ni/Mn) is 2.99, so Ni/Co/ Mn is close to 6:2:2. In the ternary cathode precursor, no Li was deter mined, and the contents of Cu, Fe, and Al were 0.796, 3.214, and 0.859 ppm, respectively. Fig. 3(c–f) shows the distribution of all the four ele ments. It could be noted that every element is evenly distributed across the micrograph. The molar ratio of LiNi0.6Co0.2Mn0.2O2 was calculated to be about 6:2:2 according to the atomic percentages of O, Ni, Co, and Mn (Table 5S), which agrees with the theoretical elemental composition of the precursor. The XRD pattern of LiNi0.6Co0.2Mn0.2O2 is shown in Fig. 4. It could be observed that the peaks perfectly match with the standard PDF card. Every diffraction peak belongs to the characteristic peak of α-NaFeO2 type with the space group of R3m, and no impurity peak was detected in the sample. This indicates the high purity of the obtained material. Different parameters of LiNi0.6Co0.2Mn0.2O2 crystal were calculated and have been shown in Table 6S. The c/a value of LiNi0.6Co0.2Mn0.2O2 is 4.932, and there are two significant bifurcations at the peaks of 006/102 and 108/110. All these suggest the presence of an excellent layered structure of LiNi0.6Co0.2Mn0.2O2 [42]. The intensity ratio (R) of I (003) and I (104) is a sensitive parameter to determine the degree of cation distribution in the crystal lattice and to measure the degree of cation mixing in the crystal of the material. It can be considered that the cation mixing is relatively light when R � 1.2. The larger the R-value, the lower the degree of cation mixing, and the more favorable the migration of lithium ions [43]. The R-value of LiNi0.6 Co0.2Mn0.2O2 is 1.13, which indicates that a part of 3a Liþ was replaced by 3b Ni2þ, and exhibits a significant relationship with the high content of Ni, and the preparation of cathode with high nickel content requires a more rigorous process [44]. 3.4. Electrochemical properties of the resynthesized LiNi0.6Co0.2Mn0.2O2
3.5. Mechanism of the synthesis of ternary cathode precursor
The cycling stabilities of LiNi0.6Co0.2Mn0.2O2 were measured at the rate of 0.2C, 0.5C, 1 C, and 3 C in a potential window ranging from 2.7 to 4.2 V (Fig. 5(a–d)). The specific capacity at the first charge and discharge at 0.2C were 196.26 mA h.g 1, and 180.072 mA h.g 1, respectively, and the coulombic efficiency was about 90%. A solid
The leaching solution contains a large amount of Ni2þ, Co2þ, and Mn , and when NaOH solution is added to the solution, the following reactions occur [47]: 2þ
Fig. 2. SEM images of the ternary cathode precursor (a) and LiNi0.6Co0.2Mn0.2O2 (b). 4
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Fig. 3. SEM-EDS images of LiNi0.6Co0.2Mn0.2O2 and elemental mapping with the corresponding mappings of single elements.
Fig. 4. XRD pattern of ternary cathode material.
Ni2þ þ 2OH ⇋ Ni(OH)2, ksp ¼ 2.0 � 10
15
Co2þ þ 2OH ⇋ Co(OH)2, ksp ¼ 1.09 � 10 Mn
2þ
þ 2OH ⇋ Mn(OH)2, ksp ¼ 1.9 � 10 2
,
13
(1)
,
(2)
,
(3)
15
In the process of the synthesis of precursor, the nickel and cobalt ions at first interact with NH3⋅H2O in a basic solution, and form Ni(NH3)2þ i (i ¼ 0, 1, 2, 3, 4, 5, and 6) and Co(NH3)2þ j (j ¼ 0, 1, 2, 3, 4, 5, and 6), yet the manganese ammonia complex ion is Mn(NH3)2þ k (k ¼ 0, 1, and 2). This reduces the concentration of metal ions in the solution and effectively prevents the rate of formation of M(OH)2 crystal nuclei. Subsequently, when ammonia water is added simultaneously with NaOH, an equilib rium system of Ni(II)–Co(II)–Mn(II)–NHþ 4 -NH3-H2O exists in the reactor. Since the solubility-product constant (Ksp) of manganese is much larger than that of nickel and cobalt, it balances the deficiency of manganese and NH3 complexation as compared to nickel and cobalt. Therefore, controlling [NH3]T ([NH3]T: total concentration of NH3) and pH can adjust the precipitation rates of Ni2þ, Co2þ, and Mn2þ to reach similar ones. P. Yang [49] calculated the thermodynamics of the system to demonstrate that the amount of ammonia affects the equilibrium of Eqns. (1)–(3). Thus, a high-performance ternary cathode precursor can be synthesized by adjusting the pH value and concentration of ammonia to effectively control the rate of precipitation and morphology of the product. With the continuous addition of NaOH, the pH of the solution in the reactor gradually rises. When the pH reaches a specific value, a large amount of OH in the solution reacts with M(NH3)2þ n complex ion to generate the M(OH)2 crystal nuclei, and the nucleus continuously grows. Finally, a spherical precursor product is obtained. The mechanism of the synthesis of ternary cathode precursor is presented in Fig. 7. As shown in Fig. 7, the mixed cathode materials of spent lithium-ion batteries were leached using sulfuric acid solution, and the ternary cathode precursor was synthesized by co-precipitation in the presence of complexing agent (ammonia water) and the precipitant sodium hydroxide.
2þ
Ni þ and Co start to precipitate at the pH values of 6.7 and 6.6, respectively, yet Mn2þ begins to precipitate at 7.8 [48]. The pH of the leaching solution is about 1.5, and the ksp values of Ni(OH)2 and Co (OH)2 differ significantly from that of Mn(OH)2. If the NaOH solution is directly added into the leaching solution, Ni2þ and Co2þ are preferen tially precipitated, and co-precipitation with Mn2þ to form ternary cathode precursor cannot be achieved. At the same time, the rate of precipitation and morphology of the product cannot be accurately controlled. The contrastive experimental results obtained without add ing ammonia water are presented in Fig. 6S(a) and (b). Fig. 6S(a) shows that the ternary cathode precursor has no regular morphology, and the specific capacity at the first charge and discharge at 0.2C (Fig. 6S(b)) were less than that observed in Fig. 5a. Thus, reducing the rate of pre cipitation of nickel-cobalt and realizing the co-precipitation of three metal ions in a strongly acidic solution is an important factor that de cides the quality of the obtained product. It is well known that the ammonia water has a strong ability to complex with nickel, cobalt, and manganese ions, and the reaction is expressed in Eqn. (4), and the sta bility constants (K) [48] are presented in Table 7S. M2þ þ nNH3 ⇋ [M(NH3)n]2þ
3.6. Sustainable recycling process for the spent lithium-ion batteries Based on the severe current environmental protection, sustainable cycling of spent lithium-ion batteries is proposed. The flow chart of this process is shown in Fig. 8. The mixture of cathode scraps could be ob tained after the spent lithium-ion batteries are discharged and dis assembled. Subsequently, the valuable metals and insoluble materials could be separated by leaching. The leaching efficiencies of nickel, co balt, manganese, and lithium could reach more than 99%. Then, the ion concentration ratio of the leachate was adjusted, and the precursor was resynthesized by co-precipitation in the presence of ammonia water and
(4) 5
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Fig. 5. Charging and discharging capacity at 0.2C, 0.5C, 1 C, and 3 C (a–d); discharge capacity at 0.2C, 0.5C, 1 C, 3 C, and 5 C (e).
LiNi0.6Co0.2Mn0.2O2 could meet the industrial requirements. The direct synthesis of the precursor in this process considerably reduced the consumption of raw material, and the rate of loss of metal elements was less than 1%. No other impurities were formed during the leaching process, and no secondary pollution was noted. Na2SO4 was obtained by evaporating the leachate after the recovery of Li2CO3. NH3 and H2O could continue to cycle in the process by condensation. Therefore, this closed-circuit cycle has the prospect of recycling of spent battery and is in line with the strategic requirements of social sustainability. 4. Conclusion In this work, a sustainable closed-loop route to recycle the mixed cathode powders from spent lithium-ion batteries has been presented. LiNi0.6Co0.2Mn0.2O2 was directly resynthesized, and the leaching effi ciencies of Ni, Co, Mn, and Li were above 99% under the optimized conditions. The ternary cathode precursor was prepared by coprecipitation after adjusting the molar ratio of ions. Li2CO3 was ob tained in the presence of a saturated sodium sulfate solution. The characterization of LiNi0.6Co0.2Mn0.2O2 was carried out using SEM and XRD. The SEM presented uniform particle size, and the XRD pattern suggested the presence of a good-layered structure of LiNi0.6 Co0.2Mn0.2O2. The electrochemical properties of the resynthesized LiNi0.6Co0.2Mn0.2O2 were determined by the charge and discharge cycle, cyclic voltammetry characteristics, AC-impedance, and rate capacity.
Fig. 6. Nyquist plot of the cell and equivalent circuit performed to fit the Nyquist plot.
sodium hydroxide. The principal role of ammonia (2:1) is to complex nickel and cobalt to control the rate of precipitation. Liþ in the filtrate was precipitated to form Li2CO3 under the action of Na2CO3, and the mixture of the precursor and Li2CO3 was calcined to obtain LiNi0.6 Co0.2Mn0.2O2. The electrochemical performance of the resynthesized 6
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Fig. 7. The mechanism of the synthesis of ternary cathode precursor.
excellent cycling stability. The cell charge/discharge at 0.2C, 0.5C, 1 C, 3 C, and 5 C for every five cycles also showed an excellent rate capacity. Declaration of competing interest The authors declared that they have no conflicts of interest to this work. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (51974180; 21978158); Shandong Provincial Natural Science Foundation, China (ZR2018MEE011); and SDUT & Zibo City Integration Development Project (2017ZBXC070). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227567. References [1] Y. Li, Y. Bai, C. Wu, J. Qian, G.H. Chen, L. Liu, H. Wang, X.Z. Zhou, F. Wu, Threedimensional fusiform hierarchical micro/nano Li1.2Ni0.2Mn0.6O2 with preferred orientation (110) plane as high energy cathode material for lithium-ion batteries, J. Mater. Chem. 4 (2016) 5942–5951. [2] J.J. Wang, X.L. Sun, Understanding and recent development of carbon coating on LiFePO4 cathode materials for lithium-ion batteries, Energy Environ. Sci. 5 (2012) 5163–5185. [3] S.B. Ni, J.L. Liu, D.L. Chao, L.Q. Mai, Vanadate-based materials for Li-Ion batteries: the search for anodes for practical applications, Adv. Energy Mater. (9) (2019) 1803324. [4] S. Florian, D. Mudit, K. Daniela, T. Michael, H. Ortal, G. Judith, M. Evan, C. Erickson, D. Ghanty, T.M. Boris, A. Doron, Stabilizing nickel-rich layered
Fig. 8. The flow-process diagram.
The specific capacity at the first charge and discharge at 0.2C were 196.26 mA h.g 1 and 180.072 mA h.g 1, respectively, which suggested the formation of a solid electrolyte interface on the cathode surface. The coulombic efficiency could reach 99% after two cycles, which presented 7
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